The conventional manner in which an automatic tensioner is installed is shown in FIG. 5. The tensioner, indicated by reference numeral 1, has a rod 4 that is slidably inserted in a cylinder 3. This cylinder 3 is fixed to a bracket 2 with a nut 3a. An idler 6 makes a rocking movement about a pivot 5. The idler 6 with which the front end of the rod 4 is in contact is pressed against a belt 7 by the rod 4 to apply a tension to it.
A conventional automatic tensioner has been proposed in Japanese Patent Laid-Open No. 126144/1984. This tensioner is now described by referring to FIG. 6, where a plunger 9 is slidably inserted in the body 8 of the tensioner. A sealed oil chamber 10 having a large diameter is formed at the bottom of the body, and contains oil. The plunger 9 is biased toward the oil chamber 10 by a spring 13 which is mounted between a snap ring 11 and the step portion 12 of the plunger 9. The ring 11 is mounted in the opening of the body 8. The plunger 9 is centrally provided with a hole 14 in which a piston 15 is inserted so as to be slidable. A small hole 17 extends from the bottom of the hole 14, so that the large oil chamber 10 is in communication with a small oil chamber 16.
In the conventional structure shown in FIG. 6, if the oil contained in the oil chambers 10 and 16 should leak, then the plunger 9 and the rod 15 would free to move their full stroke. Then, the rod 15 would hardly be pressed against the belt. Especially, if the belt is a toothed belt, positive engagement is not assured, leading to an engine trouble or other problem.